An image sensor is a semiconductor device that converts an optical image into an electric signal. Image sensors can be classified into charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors (CIS).
During the fabrication of image sensors, a photodiode can be formed in a substrate using ion implantation. As the size of photodiodes continue to get smaller for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion in an image sensor is also reduced. This results in a reduction in image quality.
Also, since the height of a stack does not reduce as much as the area of the light receiving portion reduces, the number of photons incident to the light receiving portion also decreases. This is due to diffraction of light sometimes called an Airy disk.
To address this limitation, a photodiode can be formed using amorphous silicon (Si), or readout circuitry can be formed in a silicon (Si) substrate using a method such as wafer-to-wafer bonding. Also, a photodiode can be formed on and/or over the readout circuitry (referred to as a three-dimensional (3D) image sensor). The photodiode can be connected with the readout circuitry through a metal interconnection.
In the related art, a contact plug that connects the readout circuitry and the photodiode causes a short in the photodiode.
Also, both the source and the drain of the transfer transistor are heavily doped with N-type impurities, leading to a charge sharing phenomenon. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error can be generated.
In addition, because a photo charge is not able to readily move between the photodiode and the readout circuitry, a dark current is generated and/or saturation and sensitivity is reduced.